This invention relates to nematic liquid crystal mixtures and for their use in electro-optical devices for the modulation of the transmittant or reflected light and for the colored or black-and-white rendition of numerals, symbols and moving or still pictures.
The use of liquid crystals for this purpose is known. This kind of device is based on the fact that the preferential orientation in thin layers of liquid crystals can be modified by application of an electrical field. With these modifications of the orientation goes a modification of the optical behaviour (double refraction, rotatory properties and light absorption).
Depending on the specific pretreatment of the electrodes and the initial orientation, the dielectric anisotropy, and conductivity, the dichroism obtained by addition of suitable materials and depending on the strength and direction of the applied electric field, various electro-optical effects are observed and can be made use of for technical purposes (M. Tobias: International Handbook of Liquid Crystal Displays 1975-1976, Ovum Ltd., London 1976; G. Meier, E. Sackmann, J. G. Grabmaier: Applications of Liquid Crystals, Springer-publication, Berlin-Heidelberg-New York 1975).
One prior art process, for instance, is based on the fact that by application of an electric field the rotatory property of a layer having a twisted structure may be eliminated and permeability or extinction of the introduced light between two parallel (crossed) polarizers may thus be obtained (M. Schadt, W. Helfrich: Applied Physics Letters 18, 127 (1971).
Depending on the basic electro-optical effect the compounds employed must meet certain requirements, their melting point must be far below room temperature (if possible below 0.degree. C.), their clear transparency temperatures must be above +50.degree. C., they must have a low viscosity, a high chemical and thermal stability and a stability upon continuous exposure to daylight. In addition, the materials must have a specific dielectric anisotropy, preferably a high positive anisotropy, and a low electric conductivity.
Pure compounds which meet all these requirements are not available. For this reason mixtures of different compounds which frequently belong to different classes of chemical structure are employed. Because of limited miscibility it is, however, not possible to combine any desired compounds.
It is known to employ in the manufacture of electro-optical cells, compounds from the homologous series of the phenyl pyrimidines having the general formula ##STR4## In this formula R.sup.1 and R.sup.2 signify the same or different substituents, for instance, C.sub.n H.sub.2n+1 --, C.sub.n H.sub.2n+1 O--, C.sub.n H.sub.2n+1 COO-- or C.sub.n H.sub.2n+1 CO-- and n=1 to 12.
(H. Zaschke, H. Schubert, F. Kuschel, F. Dinger, D. Demus, economic patent of the German Democratic Republic 95892).
For instance, a mixture identified herein as EQU Mi3 (A)
consists of two components of the same chemical group:
______________________________________ 5-n-hexyl-2-(4-n-hexyloxyphenyl)-pyrimidine 68.5 mol % 5-n-hexyl-2-(4-n-nonyloxyphenyl)-pyrimidine 31.5 mol % ______________________________________
This mixture has the following properties:
______________________________________ Melting temperature +4.degree. C., Clear transparency temperature 60.degree. C., DK anisotropy +0.7 Voltage requirement for the electric field effects in twisted layers: 10% voltage 6.0 V 90% voltage 8.2 V Starting time (after application of the four-fold threshold limited to a twisted layer at 25.degree. C. and a layer thickness of 20 .mu.m) 170 ms Switching off time 320 ms. ______________________________________
This mixture does not meet the requirements regarding melt behaviour and the required voltages for the electric field effect in twisted layers. The melting temperature of the thermodynamically stable, solid crystal phase should be below 0.degree. C. if possible and the 10% voltage should be below 2 V.
It has also become known to use for the making of liquid crystal displays compounds taken from the homolog series of 4-substituted-cyclo-hexanecarboxylic acid-(4-substituted-phenylesters). These compounds have the general formula ##STR5## In this formula R may be C.sub.m H.sub.2m+1, --OC.sub.m H.sub.2m+1, --COC.sub.m H.sub.2m+1, --CN or --NO.sub.2 and n, m=1-10. (H.-J. Deutscher, F. Kuschel, H. Schubert, D. Demus, Economic patent 105,701 of the German Democratic Republic).
For instance a mixture identified herein as EQU Mi14 (B)
may, for instance, consist of three components of this group of compounds as follows:
______________________________________ 4-n-propyl-cyclohexanecarboxylic acid- 34.5 mol % (4-cyanophenylester) 4-n-butyl-cyclohexanecarboxylic acid- 31 mol % (4-cyanophenylester) 4-n-pentyl-cyclohexanecarboxylic acid- 34.5 mol % (4-cyanophenylester) ______________________________________
This mixture has the following properties:
______________________________________ Melting temperature +12.5 to +16.degree. C. Clear transparency temperature 72.degree. C. DK anisotropy +6.5 10% voltage 1.4 V 90% voltage 2.1 V Starting time 115 ms Switching off time 240 ms ______________________________________
This mixture likewise does not meet the above formulated requirements insofar as its melt behavior is concerned.
For making displays with liquid crystals compounds of the homolog series of 4-substituted-benzoic acid-(4-cyanophenylester) can furthermore be used. These compounds have the general formula ##STR6## In these formulas R is C.sub.n H.sub.2n+1 -- or C.sub.n H.sub.2n+1 O-- and n=1 to 14.
(A Boller, H. Scherrer, M. Schadt: Proceedings of the JEEE 60 1002 (1972); H.-J. Deutscher, F. Kuschel, H. Bargenda, H. Schubert, D. Demus: economic patent No. 106,120 of the German Democratic Republic).
An example of these compounds is the mixture identified herein as EQU Mi16. (c)
It consists of three components of this group of compounds as follows:
______________________________________ 4-n-hexyl-benzoic acid-[4-cyanophenylester] 36 mol % 4-n-heptyl-benzoic acid-[4-cyanophenylester] 37 mol % 4-n-octyl-benzoic acid-[4-cyanophenylester] 27 mol % ______________________________________
This mixture has the following properties:
______________________________________ Melting temperature +13.degree. to 21.degree. C. Clear transparency temperature 53.degree. C. DK anisotropy about +20 10% voltage 1.7 V 90% voltage 2.3 V Starting time 420 ms Switching off time 600 ms. ______________________________________
The melt temperature of this mixture Mi 16 is too high to permit its technical use in spite of the comparatively favorable dielectric properties.
It is therefore an object of this invention to provide for compound mixtures which are chemically and thermally stable, have melting points far below room temperature and clear transparency points above 50.degree. C. and are distinguished by a particularly low voltage requirement. It is a further object of this invention to provide for the use of these mixtures of compounds in electro-optical structural elements.
A more general object of the invention is to provide for mixtures of liquid crystals which have a melting temperature and a voltage requirement lower than that of the components of the mixture.